The present invention relates in general to data storage systems such as disk drives, and it particularly relates to a read/write head, such as a thin film head, a MR head, or a GMR head for use in such data storage systems. More specifically, the present invention provides a novel design of a microactuator, such as a piezoelectric microactuator, for use in conjunction with a flexure tongue with offsetting hinges, to perform a fine positioning of the magnetic read/write head. The substantial gain in the frequency response bandwidth greatly improves the performance and accuracy of the track-follow control for fine positioning. Furthermore, the simplicity of the enhanced microactuator design results in a manufacturing efficiency that enables a high-volume, low-cost production.
In a conventional magnetic storage system, a magnetic head includes an inductive read/write transducer mounted on a slider. The magnetic head is coupled to a rotary voice coil actuator assembly by a suspension over a surface of a spinning magnetic disk.
In operation, a lift force is generated by the aerodynamic interaction between the magnetic head and the spinning magnetic disk. The lift force is opposed by equal and opposite spring forces applied by the suspension such that a predetermined flying height (or fly height) is maintained over a full radial stroke of the rotary actuator assembly above the surface of the spinning magnetic disk. The flying height is defined as the spacing between the surface of the spinning magnetic disk and the lowest point of the slider assembly.
One objective of the design of magnetic read/write heads is to obtain a very small flying height between the read/write element and the disk surface. By maintaining a flying height close to the disk, it is possible to record short wavelength or high frequency signals, thereby achieving high density and high storage data recording capacity.
The slider design incorporates an air bearing surface to control the aerodynamic interaction between the magnetic head and the spinning magnetic disk thereunder. Air bearing surface (ABS) sliders used in disk drives typically have a leading edge and a trailing edge at which read/write heads are deposited. Generally, the ABS surface of a slider incorporates a patterned topology by design to achieve a desired pressure distribution during flying. In effect, the pressure distribution on the ABS contributes to the flying characteristics of the slider that include flying height, pitch, and roll of the read/write head relative to the rotating magnetic disk.
In a conventional magnetic media application, a magnetic recording disk is comprised of several concentric tracks onto which magnetization bits are deposited for data recording. Each of these tracks is further divided into sectors where the digital data are registered.
As the demand for large capacity magnetic storage continues to grow, the current trend in the magnetic storage technology has been proceeding toward a high track density design of magnetic storage media. In order to maintain the industry standard interface, magnetic storage devices increasingly rely on reducing track width as a means to increase the track density without significantly altering the geometry of the storage media.
A smaller track width poses several mechanical and electrical problems to the operation of magnetic disk drives. One such problem lies in its actuation and control feature, which is critical to the operation of a magnetic disk drive. In order to appreciate the magnitude of this problem, it might be important to further describe the control aspect of a conventional magnetic read/write head.
In a conventional magnetic disk drive, a read/write head includes a transducer mounted on a slider. The slider is in turn attached to a stainless steel flexure. The flexure and the load beam to which the flexure is attached, form a suspension arm. The suspension arm is connected to one distal end of an actuator arm, which is driven by a voice coil motor (VCM) at the other distal end, to cause it to rotate at its mid-length about a pivot bearing.
The suspension arm exerts an elastic force to counteract the aerodynamic lift force generated by the pressure distribution on the ABS of the slider. The elastic force together with the stiffness of the suspension arm controls the stability of the actuator arm with respect to the pitch, roll, and yaw orientations. With respect to the control feature of the magnetic disk drive, during each read or write operation, there are usually two types of positioning controls: a track-seek control and a track-follow control.
A track-seek and follow control is typically commanded when data are to be retrieved from, or new data are to be written to a particular sector of a data track. Electronic circuitry incorporating an embedded feedback control software, supplies a necessary voltage to the VCM to actuate the VCM to drive the actuator arm, to which the read/write head is attached, to a target track. Thus, a track-seek control performs a low-resolution or coarse positioning of the read/write head from one data track to another data track and also following track of corresponding track pitch density
Upon the completion of a track-seek control, subsequent data operation is typically confined to within the target track. In the earlier stage of the magnetic storage technology, a typical data track is sufficiently wide so that small variations in the position of the read/write head resulting from external disturbances to the track-seek control plant do not cause the position of the read/write head to exceed the prescribed control error allowance.
As the track width reduces as a means to increase the track density and hence the storage capacity of magnetic disk drives, the foregoing single-stage actuation design encounters a significant degree of difficulty, mainly due to the excessive control error of the track-seek control using the VCM. In particular, a single-stage actuation using the VCM is found to be inadequate because the resulting control error due to external disturbances, such as inertial shock loading or noise sometimes, could cause the read/write head to be positioned over tracks that are adjacent to the target track, thus possibly causing a magnetic field disturbance of the existing data thereon.
In a worst case scenario, the data disturbances can result in a total erasure of data in the adjacent tracks after several repetitive write operations, or data corruption upon reading. Moreover, the VCM employed in a single-stage actuation is typically subjected to a mechanical resonance at the lowest natural frequency in the range of 2000 Hz-6000 Hz due to the flexibility of the actuator arm and followed by frequencies on the suspension arm in the range of 2 kHz-15 kHz.
The response of the servo-system further limits the frequency bandwidth to less than 1500 Hz. As a result, this low frequency bandwidth imposes a severe penalty on the single-stage actuation system in such a manner that the track-seek and track-follow control is unable to rapidly and precisely respond to a change in the position of the read/write head, thus causing a significant degradation in the performance of the magnetic disk drive.
To address this technical concern, it is recognized that in order to maintain the position of the read/write head in a manner that it follows a concentric path within a narrow track width of the target data track, necessary corrections to the motion of the actuator arm are required. This provision is made possible by a enhanced track-follow control, which uses a feedback on the position error signal (PES) to make an appropriate correction to the motion of the actuator arm, so as to have the position of the read/write head follow a concentric path of the target data track within a prescribed control error allowance.
Thus, in the presence of external disturbances, variations in the position of the read/write head would not cause the position of the read/write head to significantly deviate from the target position in excess of the control error allowance. To implement this track-follow control plant, a microactuator is frequently incorporated in the control feedback loop.
Various types of microactuator have been proposed, including piezoelectric (PZT) actuators, electrostatic micro-electrical mechanical systems (MEMS), and electromagnetic microactuators. By adjusting the voltage supplied to the microactuator, the track-follow control makes necessary corrections to the position of the actuator arm in the presence of external disturbances, so that the read/write head follows the target data track with a predetermined degree of precision.
The implementation of a microactuator-based high-resolution positioning in addition to the usual VCM- based single-stage actuation, is referred to as a dual-stage servo system, which is still under intensive research and development. At the time of this application is made, the microactuator-based dual stage servo systems have not been used in commercially sold drives.
A high-resolution positioning could include a rotating flexure design, and a piezoelectric rotary motor with virtual pivot. In principle, a rotating flexure design utilizes the physical properties of the piezoelectric material to convert a translational elongation or contraction of the piezoelectric material under an applied voltage to cause a distortion, which, in turn, induces a rotation of the stainless steel flexure and the hence slider that contains (or supports) the read/write head.
Notwithstanding the ability to achieve the track-follow control objective, these microactuator designs suffer from significant shortcomings among which are the following:
In certain rotating flexure microactuator designs, the piezoelectric material is arranged in a flexure beam configuration such as a cantilever. During a track-follow control actuation, the flexural (bending) deflection is converted into distortion, which causes a rotation of the flexure.
However, in relying on the flexural deflection, the piezoelectric would possess some natural modes of vibration at low frequencies, which could be easily excited by a sudden motion as commanded by the track-follow control. To minimize this excitation force, the track-follow control may command a more gradual motion to reduce the inertial force loading. In so doing, the track-follow control performance may be significantly compromised.
Still another concern with certain conventional or proposed microactuator designs, is the application of the piezoelectric material in a complex pattern. This patterning technology of the piezoelectric material has not yet reached a level of maturity with respect to manufacturing efficiency that is conducive to a high-volume, low-cost production.
As presented earlier, certain conventional rotating flexure microactuator designs rely on flexural deflections of the piezoelectric material to cause the flexure/slider assembly to rotate. This rotation is dependent upon the motion of the piezoelectric material, also known as stroke. In some instances, an amplification device is also incorporated into the design of piezoelectric material in order to effect a sufficient stroke requirement to enable the microactuator to achieve the full range of rotation of the slider. The additional amplification requirement further complicates the slider design, hence resulting in an added cost.
Yet another problem associated with a conventional microactuator design lies in the piezoelectric material itself. The output force induced by the piezoelectric material often can be substantially large that it may present itself as an excitation force to the suspension arm assembly, which typically possesses low natural frequencies. Thus, if the output force is not properly controlled, a resonance vibration of the suspension arm assembly would ensue, thereby causing an undesirable disturbance problem for the track-follow control system.
It is recognized that a further enhancement in the microactuator design for a fine positioning of the read/write head is beneficial to manufacturability, reliability and performance. Preferably, the enhanced microactuator would provide all the advantages afforded by the track-follow control system, but with a design that does not rely on a flexural arrangement of the piezoelectric material.
Furthermore, the enhanced design would achieve a controlled piezoelectric output force, and the stroke requirement without the necessity for amplification. Moreover, the enhanced microactuator design should incorporate a relatively simple piezoelectric arrangement that does not require patterning technology that typically increases the manufacturing efficiency and reduces the production cost.
It is a feature of the present invention to provide a novel enhanced microactuator for fine positioning of the read/write head during a track-follow control operation. The enhanced microactuator according to the present invention is designed to be used in a collocated dual-stage actuation servo system that substantially boosts the servo frequency bandwidth to enhance the track-seek and track-follow controls for high capacity magnetic storage device.
According to a preferred embodiment, the present invention features a novel application of piezoelectric motors in the form of a monolithic block that is suitable for low cost and manufacturing efficiency. The piezoelectric monolithic motor can be either a bulk or multi-layer type that is sandwiched between the flexure tongue and the slider. The piezoelectric motor is bonded to two hinged islands on the flexure tongue on one side, while the other side is bonded to the slider top surface.
The longitudinal axes of the hinges to which the piezoelectric motor is bonded, are further separated by an appropriate offset distance that is perpendicular to the longitudinal axis of the piezoelectric motor. A control signal commands a voltage to be supplied to the piezoelectric motor, thus causing it to either expand or contract in accordance with the voltage polarity. The equal, but opposite longitudinal forces developed within the offsetting hinges, due to the elongation or contraction of the piezoelectric motor, results in a torque that causes the piezoelectric motor as well as the slider to displace in a clockwise or counterclockwise rotation about the axis which passes through the dimple and is perpendicular to slider ABS.
Such resulting rotation demonstrates several important features for this invention, among which is that the pure resultant torque developed by the piezoelectric motor causes virtually no excitation of any load beam and flexure modes, thereby improving the accuracy of the track-follow control.
Another feature of the present invention lies in the design of the offset distance between the two hinges of the flexure. The offset distance can be adjusted to achieve simultaneously a high stroke sensitivity and a force limitation, in order to further prevent mechanical excitation of the suspension arm assembly.
Various alternative embodiments can be derived from the implemented according to the present invention. One such alternative embodiment includes two piezoelectric motors bonded to a flexure having two hinges. The two piezoelectric motors may be placed along the length or width of the slider, and may be confined within, or may extend beyond the footprint of the slider.
Still another alternative embodiment utilizes multiple piezoelectric motors (e.g., three or more) that are arranged in a circumferential configuration, such that the longitudinal axes (or force centers) of the piezoelectric motors are offset from the circular center, to impart the desired rotation.